The present invention relates to an objective lens for optical pickup employed in an optical disc drive.
In an optical pickup, a plastic objective lens is generally used for decreasing a manufacturing cost. However, change of refractive index due to temperature change, and a linear expansion coefficient of the plastic lens are relatively large in comparison with those of a glass lens. Accordingly, deterioration of an optical performance of the plastic lens due to the temperature change is relatively large.
For a low NA objective lens for an optical disc having a relatively small data density, such a deterioration is within an allowable range. However, for a high NA objective lens for an optical disc having a relatively high data density (e.g., DVD: Digital Versatile Disc), the deterioration due to the temperature change exceeds the allowable range, and it is necessary to compensate for the deterioration of the optical performance.
In Japanese Patent Provisional Publication HEI 11-337818, such a problem is dealt with as follows. When the temperature changes, the wavelength of a semiconductor laser, which is a light source, also changes. According to the publication, a minute diffraction lens structure is formed on a surface of a refraction lens so that spherical aberration varies when the wavelength of the beam changes, and the deterioration of the performance due to the temperature changes is absorbed by the change of the spherical aberration due to the change of the wavelength of the light source. Such a lens is typically manufactured by forming the diffraction lens pattern in a metal molding, and in accordance with an injection molding method using the metal molding.
The diffraction lens structure has a number of zones, and therefore, it is relatively troublesome to process the metal molding. Further, since the width of the zones is smaller at a peripheral area of the lens surface, a minute cutting tool should be used. The minute cutting tool is, however, easy to be worn out, and thus the cutting tools should be exchanged frequently. Thus, the manufacturing cost increases if the configuration described above is employed.
The present invention is advantageous in that the deterioration of the optical performance of the objective lens is well suppressed, and further, processing of a metal molding for such an objective lens is easy and the manufacturing cost can be lowered.
According to an aspect of the invention, there is provided an objective lens for an optical pickup having a single lens element having two refraction surfaces. One of the two refraction surfaces is divided into a central area including an optical axis of the objective lens and a peripheral area outside the central area, and a step providing a level difference along a direction of the optical axis being formed at a boundary between the central area and the peripheral area. The step providing a phase shift between light passing through the central area and light passing through the peripheral area. The phase shift suppresses deterioration of wavefront aberration due to a change of temperature. The level difference is formed such that a thickness on the peripheral area side is greater than a thickness on the central area side at the boundary, the objective lens being configured to satisfy a condition:
0.83 less than hx/hmax less than 0.97,
wherein,
hx is a radius of the boundary, and
hmax is an effective radius defining a numerical aperture of the surface formed with the step.
Optionally, the objective lens may be configured to satisfy a condition:
0.88 less than hx/hmax less than 0.93.
Further optionally, the step may be formed to satisfy condition:
xe2x88x9214 less than Nxc3x97xcex less than 0 [xcexcm],
where, N is an optical path difference (unit: xcex) of a ray passing through the peripheral area with respect to a ray passing through the central area, and
xcex is a design wavelength (unit: xcexcm).
Still optionally, the step may be configured to satisfy a condition:       0.07     less than                             N          xc3x97          λ                          NA          4                    xc3x97              (                                            λ              ⁡                              (                                                      n                    xe2x80x2                                    -                  1                                )                                                                    λ                xe2x80x2                            ⁡                              (                                  n                  -                  1                                )                                              -          1                )               less than           1.20      ⁢              xe2x80x83            [      μm      ]        ,
where, N represents an optical path difference (unit: xcex) of a ray passing the peripheral area with respect to a ray passing the central area,
NA represents a numerical aperture of the objective lens,
xcex represents a design wavelength [xcexcm],
xcexxe2x80x2 represents a wavelength at a changed temperature [xcexcm],
n represents a refractive index of material of the objective lens at the design wavelength, and
nxe2x80x2 represents a refractive index of the material of the objective lens at a wavelength at the changed temperature.
Optionally, the step may be formed to satisfy a condition:   0.22   less than                     N        xc3x97        λ                    NA        4              xc3x97          (                                    λ            ⁡                          (                                                n                  xe2x80x2                                -                1                            )                                                          λ              xe2x80x2                        ⁡                          (                              n                -                1                            )                                      -        1            )         less than             0.62      ⁢              xe2x80x83            [      μm      ]        .  
In the above cases, it may be convenient if N is an integer.
Further optionally, the objective lens may be formed using a molding which is processed using rounded corner cutting tool.
According to another aspect of the invention, there is provided an objective lens for an optical pickup having a single lens element having two refraction surfaces. One of the two refraction surfaces is divided into a central area including an optical axis of the objective lens and a peripheral area outside the central area, and a step providing a level difference along a direction of the optical axis is formed at a boundary between the central area and the peripheral area. The step provides a phase shift between light passing through the central area and light passing through the peripheral area, the phase shift suppressing deterioration of wavefront aberration due to a change of temperature, the level difference being formed such that a thickness on the central area side is greater than a thickness on the peripheral area side at the boundary. The objective lens is configured to satisfy a condition:
0.34 less than hx/hmax less than 0.60,
where hx is a radius of the boundary, and
hmax is an effective radius defining a numerical aperture of the surface formed with the step.
Optionally, the objective lens may be configured to satisfy a condition:
0.41 less than hx/hmax less than 0.55.
Optionally, the step is formed to satisfy a condition:
0 less than Nxc3x97xcex less than 12 [xcexcm],
where N is an optical path difference (unit: xcex) of a ray passing through the peripheral area with respect to a ray passing through the central area, and
xcex is a design wavelength (unit: xcexcm).
Further optionally, the step is configured to satisfy a condition:             -      1.20         less than                             N          xc3x97          λ                          NA          4                    xc3x97              (                                            λ              ⁡                              (                                                      n                    xe2x80x2                                    -                  1                                )                                                                    λ                xe2x80x2                            ⁡                              (                                  n                  -                  1                                )                                              -          1                )               less than           -              0.07        ⁢                  xe2x80x83                [        μm        ]              ,
where N represents an optical path difference (unit: xcex) of a ray passing the peripheral area with respect to a ray passing the central area,
NA represents a numerical aperture of the objective lens,
xcex represents a design wavelength [xcexcm],
xcexxe2x80x2 represents a wavelength at a changed temperature [xcexcm],
n represents a refractive index of material of the objective lens at the design wavelength, and
nxe2x80x2 represents a refractive index of the material of the objective lens at a wavelength at the changed temperature.
Still optionally, the step may be formed to satisfy a condition:       -    0.05     less than                     N        xc3x97        λ                    NA        4              xc3x97          (                                    λ            ⁡                          (                                                n                  xe2x80x2                                -                1                            )                                                          λ              xe2x80x2                        ⁡                          (                              n                -                1                            )                                      -        1            )         less than       -                  0.22        ⁢                  xe2x80x83                [        μm        ]            .      
It may be convenient if N is an integer.
Further optionally, the objective lens may be formed using a molding which is processed using an R cutting tool.